1. 1.Introduction.
Physics is the study of events in the universe and the interaction between objects.
Physics is the science of nature's most fundamental and has many phenomena and natural
phenomena that can be used as a basic science.
Physics have often learned in junior high school and senior high school.Ever you hear
about newton’s law,its like newton’s third law?Of course yes!F action= - F reaction.Its the
answer that often we hear if some one answering when we asked about newton’s third law.This
answer not only wrong meaning cause the answer is same with the mathematic equation.And
then this argument make a misconception in this topic or material.
We wont remember action reaction formula cause physics lesson isn’t just to remember
step by step or like remember or memorize the song.We will introduction with newton’s law use
the known concept and the aplication in our daily life.To minimalize the misconception in this
material,Let’s we explain about it.
2. 2.Misconception in newton’s law
a.action reaction force act on the same body.
b.there is no connection between newton’s law and kinematics.
c.the product of mass and acceleration,ma,is a force.
d.friction can’t act in the direction of motion.
e.the normal force on an object is equal to the weight of the object by the third law.
3.Theoritical observation.
Newton’s first law
Newton’s first law often we call with inertia law(kelembaman).Characteristic of weak
object is the characteristic to defent condition it,that is a constant condition.if static object
condition or dynamic condition.it’s like for the car that has initial condition static,will like the
initial condition if we didn’t give force to it.the dynamic object with certain velocity,will
move/do the motion with constant velocity without force that disturb the object.its same with
newton’s first law:
If all force that work at an object is equal to zero,so that an object or static object will be
static,and the object that do the motion with constant velocity will do the motion with constant
velocity.with mathematic,we can write:
(SIGMA)F = 0
Newton's first law of motion is often stated as
An object at rest stays at rest and an object in motion stays in motion with the same speed
and in the same direction unless acted upon by an unbalanced force.
There are two parts to this statement - one that predicts the behavior of stationary objects
and the other that predicts the behavior of moving objects. The two parts are summarized in the
following diagram.
3. The behavior of all objects can be described by saying that
objects tend to "keep on doing what they're doing" (unless
acted upon by an unbalanced force). If at rest, they will
continue in this same state of rest. If in motion with an
eastward velocity of 5 m/s, they will continue in this same
state of motion (5 m/s, East). If in motion with a leftward
velocity of 2 m/s, they will continue in this same state of motion (2 m/s, left). The state of motion
of an object is maintained as long as the object is not acted upon by an unbalanced force. All
objects resist changes in their state of motion - they tend to "keep on doing what they're doing."
There are many more applications of Newton's first law of motion. Several applications
are listed below. Perhaps you could think about the law of inertia and provide explanations for
each application.
Blood rushes from your head to your feet while quickly stopping when riding on a
descending elevator.
The head of a hammer can be tightened onto the wooden handle by banging the bottom of
the handle against a hard surface.
A brick is painlessly broken over the hand of a physics teacher by slamming it with a
hammer. (CAUTION: do not attempt this at home!)
To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down
and thrusted downward at high speeds and then abruptly halted.
Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.
While riding a skateboard (or wagon or bicycle), you fly forward off the board when
hitting a curb or rock or other object that abruptly halts the motion of the skateboard.
4. Newton’s second law
Newton’s second law will explain about condition of object if force resultant that work
not zero.imagine it, if you pull the object in wet floor,the object will move with a acceleration.In
experiment result,if we make a change in the force like add the force so that the acceleration will
be change equal with the force change.
The conclusion of the experiment can be complete in newton’s second law:
The acceleration of the object is equal to or pendicular with force resultant that work,and its
perpendicular with the mass.
In mathematic equation,we can write:
(SIGMA)F=m.a
(SIGMA) F=force resultant
M=object mass
A=acceleration
In vector equation,also can write:
Fx=m.ax
Fy=m.ay
Fz=m.az
Newton's second law of motion pertains to the behavior of objects for which all existing
forces are not balanced. The second law states that the acceleration of an object is dependent
upon two variables - the net force acting upon the object and the mass of the object. The
acceleration of an object depends directly upon the net force acting upon the object, and
inversely upon the mass of the object. As the force acting upon an object is increased, the
acceleration of the object is increased. As the mass of an object is increased, the acceleration of
the object is decreased.
Newton's second law of motion can be formally stated as follows:
The acceleration of an object as produced by a net force is directly proportional to the magnitude
of the net force, in the same direction as the net force, and inversely proportional to the mass of
the object.
This verbal statement can be expressed in equation form as follows:
a = Fnet / m
5. The above equation is often rearranged to a more familiar form as shown below. The net
force is equated to the product of the mass times the acceleration.
Fnet = m * a
In this entire discussion, the emphasis has been on the net force. The
acceleration is directly proportional to the net force; the net force equals mass
times acceleration; the acceleration in the same direction as the net force; an
acceleration is produced by a net force. The NET FORCE. It is important to
remember this distinction. Do not use the value of merely "any 'ole force" in the
above equation. It is the net force that is related to acceleration
Newton’s third law
Definition of newton’s third law:
If an object working force at the second object,so that the second object working the first
object force,that total of the force=receiving force but different direction.
In mathematic equation,we can write:
Faction=-Freaction
Faction =The force that work at object
Freaction=object reaction force effect action force
6. Some forces result from contact interactions (normal, frictional, tensional, and applied
forces are examples of contact forces) and other forces are the result of action-at-a-distance
interactions (gravitational, electrical, and magnetic forces). According to Newton, whenever
objects A and B interact with each other, they exert forces upon each other. When you sit in your
chair, your body exerts a downward force on the chair and the chair exerts an upward force on
your body. There are two forces resulting from this interaction - a force on the chair and a force
on your body. These two forces are called action and reaction forces and are the subject of
Newton's third law of motion. Formally stated, Newton's third law is:
For every action, there is an equal and opposite reaction.
The statement means that in every interaction, there is a pair of forces
acting on the two interacting objects. The size of the forces on the first object
equals the size of the force on the second object. The direction of the force on
the first object is opposite to the direction of the force on the second object.
Forces always come in pairs - equal and opposite action-reaction force pairs.
Friction force
Friction force same with normal force,this force there if the surface of two object life
touch(fisic touch)direction of the friction same with touch surface and not same with motion
direction.Static friction force has maximum limit,maximum value pendicular with normal force
and static friction coefisien
In mathematic equation,we can write:
Fmax=Ms.N
If:
-kinetics
Fk=Ms.N
-static
Fs=Ms.N
7. 4.Explanation
a.Action force and reaction force work at same object
Same with newton’s second law:
‘when the object giving force to the second object,the second object giving some force
but different direction with first object.’
Important to remember if action force and reaction force work at different object.
For the example:
a.if our hand pull the table(the force vector direction to right down),the table will pull you back
too(this vector painted with different direction)
b.if we walk on the floor,usually our foot will pull the floor to the behind,and then the floor pull
our foot to the in front of derection.
b.There is no connection between newton’s law and kinematics.
In newton’s law application same with kinematics.we often find the problems that
connecting between newton’s law with kinematics.because in the application of newton’s law we
also explain about the acceleration,distance,and velocity that all variable inside of kinematics.
For example:
A car with mass 10.000 kg,move with velocity 20m/s.The car will be stop after the
distance 200m.the force of car at the car stop is?
Solution
M=10.000kg
V0=0m/s
V=20m/s
9. So that
F=m.a
F-Ff=m.a
d.Friction cant act in the direction of motion
Direction of friction same with touch object surface and not same with motion
direction.There are two friction force:
a.static friction force
b.kinetic friction force
Friction force between object surface that move with push object will make friction force
that the direction not same with object motion direction.
There are two kinds of friction force are:
Static friction force(fs):
Work at static object(stop) with the equation:
Fs=N.Ms
Kinetic friction force(fk):
Work at motion object with the equation:
Fk=N.Mk
Value of fk<fs
10. e.the normal force on an object is equal to the weight of object by the newton’s third law
total force at an object in static condition is zero,this argument same with newton’s third law.and
then the gravitation force with down direction at object must be confidence with the up force
(normal force)its force not action reaction force that same with newton’s third law.
11. 5.Conclusion
All of newton’s law is the basic classical to describe the motion
The newton’s first law(inertia law)describe about an object will be move or do the motion
in static condition if the resultant of the force not work to that’s object.
The newtons second law explain about object acceleration pendicular with the total force
and perpendicular with mass.
The newton’s third law explain about if an object working force at the second object,so
that the second object working the first object force,that total of the force=receiving force but
different direction.
In mathematic equation,we can write:
Faction=-Freaction
13. CONTENT ANALYSIS IN SCHOOL PHYSICS
“MISCONCEPTION IN NEWTON’S LAW”
BY:
ZAKIRMAN
01951/08
PHYSICS EDUCATION
LECTURER:Dra.YURNETTI,M.Pd
PHYSICS DEPARTMENT
MATHEMATIC AND NATURAL SCIENCE FACULTY
PADANG STATE UNIVERSITY
